The ability of B cells to respond to antigen relies on signals transmitted through the B cell antigen receptor (BCR) complex. Activation of cytoplasmic protein tyrosine kinases (PTKs) is the earliest measurable biochemical response to BCR cross-linking. The initial event leads to the generation of secondary signals including Ras activation, phosphatidylinositol 3-kinase (PI-3K) activation, phospholipase C (PLC)-γ2 activation.While much has been learned as to the relationship between the BCR-associated PTKs and downstream effectors, the molecular mechanism by which these PTKs regulate downstream events remains unclear. Analogous to receptor tyrosine kinases, it has been thought that many signaling molecules directly bind phosphorylated tyrosine residues on the cytoplamic domains of the BCR complex. However, this does not appear to be a feature of the coupling mechanism to downstream signaling pathways. Attention instead has focused on adaptor proteins.Based upon our previous evidence that Syk, among the BCR-associated PTKs, is essential for PLC-γ2 activation, we purified tyrosine-phosphorylated proteins mediated by Syk. Among several purified proteins, four internal peptide sequences obtained from microsequencing of pp80 were very homologous to those of human and mouse BLNK. To address the function of BLNK, we established DT40 B cells deficient in BLNK by genetargeting method. In contrast to wild-type DT40 cells, no PLC-γ2 activation was detected in BLNK-deficient cells. Taken together, we conclude that BLNK functions as an adaptor molecule in BCR signaling, which is required for coupling Syk to PLC-γ2 activation.